The present invention relates to a power capacitor of the type described in the preamble to claim 1. The power capacitor in accordance with the invention is primarily intended for a rated voltage exceeding 1 kV, e.g. 5 kV, preferably at least 10 kV.
Power capacitors are important components in systems for the transmission and distribution of electric power. Power capacitor installations are used primarily to increase the power-transmission capability through parallel and series compensation for voltage stabilisation by means of static var-systems and as filters for the elimination of harmonics.
Second and third aspects of the invention relate to use of the type described in claim 21, and to a method of the type described in claim 23.
Capacitors have a phase angle close to 90xc2x0, and therefore generate reactive power. By connecting capacitors in the vicinity of the components that consume reactive power, the desired reactive power can be generated there. Cables can thus be utilised to the full for transmitting active power. The consumption of reactive power in a load may vary and it is desirable to constantly generate a quantity of reactive power corresponding to the consumption. For this purpose, a plurality of capacitors are connected via series and/or parallel connection in a capacitor bank. The number of capacitors required to correspond to the consumed reactive can be connected in. Compensating for consumed power by utilising capacitors in the manner described above is known as phase compensation. For this purpose a capacitor bank in the form of a shunt battery is arranged in the vicinity of the components consuming reactive power. Such a shunt battery consists of a plurality of capacitors connected together. Each capacitor comprises a plurality of capacitor elements. The structure of such a conventional capacitor is described below.
A shunt battery usually comprises a number of chains of a plurality of capacitors connected in series. The number of chains is determined by the number of phases, usually three. The first capacitor in a chain is thus connected to a cable for transmitting electric power to the consuming component. The cable for transmitting is arranged a certain distance from the ground or from points in the surroundings with earth potential. This distance is dependent on the voltage of the cable. The capacitors are then connected in series from the first capacitor, which is connected to the cable, and downwards. A second capacitor arranged at the opposite end of the chain of series-connected capacitors is connected to earth potential or to a point in the electrical system having zero potential (e.g. non-earthed 3-phase system). The number of capacitors and their design are determined so that the permissible voltage (rated voltage) over the series-connected capacitors corresponds to the voltage of the cable. A plurality of capacitors are therefore series-connected and arranged in stands or on platforms insulated from earth potential. Such a capacitor bank thus includes a plurality of different components and requires relatively large quantities of material. It also requires a relative robust construction so that the stand/platform can withstand the effects of wind, earthquakes, etc. Considerable work is thus required to construct such a capacitor bank. This problem is particularly noticeable when the capacitor bank consists of a large number of capacitors. The capacitor bank also takes up a relatively large area on the ground.
Long cables for alternating voltage are inductive and consume reactive power. Capacitor banks for series-compensation are therefore arranged with regular spacing along such a cable in order to generate the necessary reactive power. A plurality of capacitors is connected in series to compensate the inductive voltage drop. In a capacitor bank for series-compensation, as opposed to a shunt battery, the series-connection of capacitors usually only takes up part of the voltage in the cable. The chains of series-connected capacitors included in the capacitor bank for series compensation are also arranged in series with the cable to be compensated.
A conventional capacitor bank comprises a plurality of capacitors. Such a capacitor in turn comprises a plurality of capacitor elements in the form of capacitor rolls. The capacitor rolls are flattened and stacked one on top of the other to form a stack 1 m tall, for instance. A very large number of dielectric films with intermediate metal layers will be arranged in parallel in the vertical direction of the stack. When a voltage applied over the stack increases, the stack will be compressed somewhat in vertical direction, due to Coulomb forces that act between the metal layers. For the same reason, if the voltage decreases the stack will expand somewhat in vertical direction. The stack formed has a specific mechanical resonance frequency or natural frequency, which is relatively low. The mechanical resonance frequency of the stack is amplified by specific frequencies of the current, which may produce a loud noise. The mains frequency constitutes such a frequency. However, amplification of the mechanical resonance frequency can also be effected by harmonics in the current.
An example of a power capacitor of this known type is described in U.S. Pat. No. 5,475,272. A high-voltage capacitor constructed from a plurality of capacitor elements stacked one on top of the other and placed in a common container, is thus described here. The container is made of metal in conventional manner. The electrical lead-throughs are made of porcelain or polymer. The publication also describes various alternative couplings for connecting the capacitor elements in series or in parallel.
In known capacitors of this type the capacitor elements are impregnated with oil. The oil is also arranged to surround the capacitor elements and thus fill up the space between these and the wall of the container. Oil is satisfactory from the insulation aspect but entails a number of drawbacks. Damage to the container or defective sealing may result in oil leakage which may damage the function of the capacitor as well as contaminating the environment.
Against this background, the object of the present invention is to overcome the problem of oil leakage from a power capacitor of the type under consideration.
From a first aspect of the invention this object is achieved by a power capacitor of the type described in the preamble to claim 1 comprising the characteristic features defined in the characterizing part of the claim. The insulating medium in the form of a dielectric fluid, e.g. an oil comprising a gelling component. The dielectric fluid may be electrically insulating oil to which gelling components have been added. In this context it should be understood that the component may consist of a mixture of part-components. The gel surrounding the capacitor elements in the container thus replaces the oil normally used for this purpose. Any damage to the container will not therefore result in oil leakage since no liquid oil is present. The consistency of the dielectric fluid prevents the formation of drops and it is therefore unable to leak out. Since the container is made of a polymer material and therefore yields to a certain extent and is negligibly sensitive to cracking, it has properties of significance in combination with the enclosed gel. The material combines good insulation ability with other desired features such as strength, manageability and cost. A design in accordance with the invention also offers favourable conditions for overcoming the problem of thermal conduction and insulation around the edges of the capacitor windings, which is a particular problem with power capacitors for high voltage.
It is known per se to gel an oil for use in electrical arrangements. PCT/SE 98/02314, for instance, describes the arrangement of an electrical arrangement comprising an electric conductor and an insulation system with a porous, fibre-based or laminated structure. The structure is impregnated with a dielectric fluid that is caused to solidify to a gel. The publication describes, inter alia, an application for impregnating a capacitor bank wound from metal and plastic foil. However, a capacitor element impregnated in this way does not eliminate the problem of leakage from the oil surrounding the capacitor elements in a container. This is because said arrangement describes a gel system in which the oil is thermo-reversible, i.e. at high temperature it becomes fluid. Neither does the publication solve this type of problem.
Additional examples are described in JP 716 12 68 and JP 103 26 721. However, this does not deal with power capacitors for high voltage either. JP 103 26 721 shows a capacitor in which the gel is intended to suppress mechanical vibrations. The object is thus completely different from that of the present invention, which is focused on the task of avoid an insulating fluid leaking out through the container. JP 103 26 721 shows a capacitor in which one side wall consists of urethane resin. The purpose is to prevent electrically conducting material penetrating out if the capacitor breaks, by avoiding cracks in the material through the addition of a more flexible material in the form of a gel. Here, too, it is a question of the gel being intended to achieve mechanical suppression.
In a preferred embodiment of the power capacitor in accordance with the invention the gel state of the dielectric fluid is thermostable throughout the entire temperature range occurring when the capacitor is in operation. Increased security against the occurrence of oil leakage is obtained by choosing the gelling component so that the gel state is retained even at relatively high temperatures. In accordance with a preferred embodiment the dielectric fluid is silicon-based, this applying in particular to the gelling silicon component. A capacitor is thus achieved which is extremely advantageous from the environmental aspect, for instance. A gel system that instead contains components such as polyurethane and/or isocyanates does not have such environmental advantages. Since these produce toxic gases in the event of a fire, they contribute to a hazardous working environment during manufacture and demands for safe waste management and destruction. Toxic gases are produced in the event of fire in a capacitor containing oil in accordance with said PCT/SE 98/02314. Furthermore, a gel system with such components has the drawback that these swell greatly and negatively influence the metallising film. Since an embodiment with metallised film, i.e. metal-coated film, is advantageous, this is a considerable drawback. Tests have shown that the films may even be destroyed. These drawbacks are avoided with a silicon-based gel system. This is therefore an embodiment of great significance.
The present invention is particularly advantageous for application in a power capacitor which is produced in known manner from capacitor elements in the form of rolled film of plastic and metal or a metal-coated film, wherein the gel is arranged to impregnate the wound capacitor element, possibly at its end portions, in order to avoid partial discharges. This thus constitutes a preferred embodiment of the power capacitor in accordance with the invention. Alternatively, such a winding can be performed dry.
In an alternative embodiment of such a power capacitor, a second dielectric fluid is arranged in the space between turns of the winding, which second dielectric fluid is in liquid form, i.e. not gelated.
The gel surrounding the capacitor elements in the container should fulfil certain requirements. It should thus display high shearing strength in gelated state, good thermal conductivity, high electric strength, be sufficiently electrically insulating and be thermostable within the temperature range occurring during operation.
In accordance with a preferred embodiment the dielectric fluid comprises an electrically insulating oil. The fluid is thus of a type that in high degree is capable of fulfilling said requirements. From this aspect, it is particularly suitable for the oil to comprise silicon oil.
In accordance with a preferred embodiment of the invention the gelling component comprises silicon, preferably polydimethyl siloxane with at least some vinyl substitutes, i.e. vinyl side groups.
In accordance with another preferred embodiment, the gelling component comprises silane-functional cross-linking agent. In a preferred alternative this cross-linking agent comprises silicon, suitably polydimethyl siloxane, with at least some silane substitutes.
The quantity of silane-functional cross-linking agent is preferably 1-80 per cent by weight.
The preferred gelling component, the preferred alternative thereof and the preferred content thereof contribute to the dielectric fluid acquiring favourable properties as regards the above requirements.
In a particularly preferred embodiment the dielectric fluid also comprises metal complex, which further contributes to satisfying the above requirements. Here, too, the quantity of metal complex mixed in is 2-4000 ppm, preferably 10-2000 ppm, which has been found to constitute a suitable amount.
In accordance with yet another preferred embodiment the dielectric fluid comprises silicon liquid of low molecular weight, preferably polydimethyl siloxane liquid. In this case also, a fluid is obtained that in gelled state satisfactorily fulfils the requirements set.
In accordance with another embodiment of the invention the dielectric fluid comprises an agent that retards gelation. This permits a well controlled and extended gelling process that facilitates manufacture and contributes to achieving good quality of the gel function.
A suitable quantity of the gelation-retarding component is 0.001-4 per cent by weight. In accordance with another preferred embodiment the composition of the dielectric fluid is 1-80 per cent by weight, preferably 20-50 per cent by weight, silane-functional cross-linking agent, 2-4000 ppm, preferably 10-2000 ppm metal complex, 0-60 per cent by weight, preferably 10-50 per cent by weight polydimethyl siloxane of low molecular weight, 0-4 per cent by weight gelation-retarding agent and the remainder polydimethyl siloxane with vinyl substitutes.
With such a composition the fluid acquires very suitable properties for insulating medium that fulfils the necessary requirements.
In accordance with an alternative, also preferred, embodiment the dielectric fluid comprises a vegetable oil, possibly mixed with silicon oil.
In accordance with a further preferred embodiment the gelling component comprises a vegetable oil.
In accordance with yet another embodiment of the invention at normal operating temperature the pressure in the gel is at least equivalent to atmospheric pressure.
In accordance with a preferred embodiment each capacitor element is substantially circular-cylindrical in shape and the inside of the container has corresponding circular-cylindrical shape so that the container closely surrounds each capacitor element, the axial direction of each capacitor element being oriented to coincide with the axial direction of the container.
Since the inside of the container has a circular-cylindrical shape corresponding to the cylindrical shape of the capacitor elements so that the container closely surrounds the capacitor elements, a capacitor is obtained that is as compact as possible and suited to an advantageous and electrically favourable shape of the elements from a manufacturing point of view.
In accordance with another embodiment the container is made of an electrically conducting material. The insulation between the capacitor elements and the container can therefore be simpler without risk of discharge between capacitor elements and container. Furthermore, the electrical connections of the capacitor can be made extremely simple and the creepage distance necessary between them can be provided by the container itself. With the simplification of the insulation and elimination of the lead-throughs, the capacitor will also be relatively compact, thereby enabling compact capacitor banks to be built.
A second aspect of the invention relates to the use of a gelled dielectric fluid to insulate capacitor elements arranged in a container. In preferred embodiments the gel has a composition corresponding to that stated above for the power capacitor in accordance with the invention. Similar advantages as those described above with regard to the invented power capacitor are gained with the use in accordance with the invention.
From a third aspect the object is achieved by means of the method defined in claim 23. A power capacitor in accordance with the invention is obtained in a practical way by means of this method.
In accordance with a preferred embodiment of the method in accordance with the invention the dielectric fluid is degassed before being introduced into the container. This increases the functional reliability of the capacitor since air bubbles are avoided in the gel, which might cause the appearance of surface glow. Such surface glow can cause erosion in the long run.
The above and other preferred embodiments of the power capacitor in accordance with the invention, the invented use and the invented method are defined in the sub-claims to respective claims 1, 21 and 23.